The invention relates to photographic printing, specifically to the calibration of color filtration in the enlarger head to provide proper color values in a finished print, corresponding to the characteristics of the printing paper used.
In the conventional art, color printing is a long and laborious process, especially calibrating the color levels in the enlarger to the printing paper to produce a color print that replicates, as closely as possible, the colors in the scene actually photographed.
Each batch of photographic paper has its own idiographic filtration characteristics, requiring that the color filtration in the enlarger be specifically calibrated for the batch of printing paper which the photographer wishes to use. This process can be extremely laborious, requiring several steps. In the conventional art, a test print is made, using a standard negative, with the filtration levels recommended in the directions accompanying the photographic paper. For example, in printing from a color negative, the recommended level is 40M+50Y, the magenta level on the enlarger will be set at 40 and the yellow level at 50 (the cyan level is rarely adjusted). A test print will then be exposed and developed, using standard methods. The photographer then views the resulting test print through colored viewing filters until the resulting image appears to be in proper color balance. The photographer then adjusts the filtration levels on the enlarger (normally only the yellow and magenta levels) and makes a new test print. In addition, the density of light is adjusted by changing the exposure time to darken or lighten the resulting print. Once the second test print is made, the photographer again views the resulting print through viewing filters and again adjusts the color filtration levels on the enlarger to obtain a color balance that appears to more closely resemble that of the scene originally photographed. Eventually this process results in an acceptable calibration, although the process can require up to seven or eight iterations. It should be noted that this process can take a large amount of time, since each iteration can take approximately 15 minutes. This is time-comsuming work for the hobbyist who must expend an hour or more before beginning to print pictures. For the professional photographer or for the processing lab which makes prints for customers, a more efficient method of calibrating color levels than is now in use would save on professional time and labor costs.
As will be shown, the present invention uses electronic means to reduce the number of steps required to calibrate an enlarger for the particular printing paper used. It is, therefore, an objective of the present invention to save time for the photographer or photographic lab technician. In professional or commercial situations, this time saving also represents a saving of money.
The method described here also allows for easier visual inspection of negatives for which color correction is difficult, for such reasons as age, film deterioration or off-color characteristics of the film originally used. The present invention vastly simplifies the process of calibration for such negatives and allows an acceptable-quality print with much less work than is now required to produce it. Accordingly, a further objective of this invention is to facilitate adjustment of color levels for "difficult" negatives. This is especially important when producing composite prints when may contain several such "off-color" negatives.
In short, the system described here introduces the substitution of a Charge Coupled Device (CCD) TV camera for the printing paper in the enlarger for the purpose of visual inspection a standard TV (CRT) monitor of the color values that would result if the image in the enlarger were to be printed. The system can reverse the polarity of a negative to display a positive image on the screen. This is accomplished by reversing the relationship between black and white and inverting the primary colors; red to cyan, green to magenta and blue to yellow. Use of a waveform monitor facilitates elimination of "off color" information, making adjustment of color values for a specific negative much easier than in the conventional practice. Polarities are not reversed for color transparencies.
This system operates in a different field from the prior art. Much of the art in the color photography field concerns chemical advances for photographic use (e.g. Viola: U.S. Pat. No. 4,304,837; Vary: U.S. Pat. No. 3,764,330) or new advances in electrophoretic photography, such as used in color copy machines (e.g. Hiranuma; U.S. Pat. No. 4,580,889). The method claimed here uses conventional photochemical methods and provides for a method of preparation for printing that is much more efficient than the methods now in use.
Other methods recently introduced into the art add color for artistic (Purro: U.S. Pat. No. 4,455,369) or illustrative (Edwards: U.S. Pat. No. 4,714,334) purposes. The system described here does not add color; it helps to replicate the color that was present in the scene as originally photographed. Moreover, it does this without resorting to new chemistry (see Giorgi: U.S. Pat. No. 4,717,646 (use of anhydrous diacetone rinse)). Other prior art adds to traditional darkroom technique, but not in the same way as the invention claimed here. Kirby U.S. Pat. No. (3,685,900) teaches an exposure calculator, but his method deals with density and not color values. Brault U.S. Pat. No. (4,081,277) teaches filter layers over light sensors but does not provide for adjustment of the color levels in the enlarger to facilitate color printing.
Even the most technically-complex advances in color reproduction differ markedly from the invention described here. Fukuchi U.S. Pat. No. (4,825,246) teaches a new method of color processing by breaking a color image down into complementary red and cyan components. That method, however, requires a complicated apparatus for its practice. The Wittocx system U.S. Pat. No. (4,810,602) photographs images shown on a TV monitor, broken down into their red, blue and green components. Each color separation is photographed through an appropriate filter to produce a corrected image. While the Wittocx system produces accurate color for such purposes as medical illustration, the system described here is designed to enhance the efficiency of the practice of the photographer's art, not to replace it with a precision that is unnecessary and undesirable. The system described here does not encompass the photographing of a CRT image; it uses such an image for calibration purposes before a negative is printed. Okino U.S. Pat. No. (4,810,603) teaches a complex method of correcting color tonality that requires filters, mirrors, digitizing circuits and feedback loops with the necessity of a new and complex apparatus. In the present system, there is no need for complicated apparatus. In the Merlo system U.S. Pat. No. (4,259,423) a wheel containing filters of the three primary colors breaks down an image and digital circuitry is used to adjust exposure time. Again, the process requires specific apparatus for its practice. The system described here processes only analog information and requires no new circuitry or logic.
It should again be noted that the objective of the invention described here is to render the practice of the photographic art more efficient than it is now by using available instruments in a novel manner. In effect, the method claimed here uses techniques associated with television technology to streamline a labor-intensive process which forms a necessary part of the photographer's work.